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What Is the 'Green Flash' at Sunset — and How Can You See It?

Sarah Mohamad
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An image of the bright sun setting on the horizon. Some distortion is seen with hints of orange and yellow. Hints of green are seen at the upper side of the sun. A silhouette of a bird is seen flying across the sun.
Green flash photographed in Half Moon Bay, California, on July 8, 2021 (Jan Null)

Have you heard of “the green flash”?

This elusive optical effect that happens during a sunset is the subject of debate online — with some people claiming it doesn’t even exist. And if you have never seen it yourself, the idea of a startling burst of green suddenly appearing next to the setting sun could sound far-fetched.

Luckily, Bay Area meteorologist and photographer Jan Null has been documenting his green flash sightings on social media. And he’s here to tell you: it is absolutely real.

What is the ‘green flash’?

Like rainbows or mirages, a green flash during sunset is another example of an optical phenomenon that occurs regularly in our daily lives yet can seem magical when you witness one yourself.

Similar to how rainbows appear when sunlight is scattered through raindrops, green flashes during sunsets — and sometimes sunrises — happen when light passes through a thick layer of Earth’s atmosphere. As the sun’s light moves through, it gets bent or refracted, creating a stunning, colorful sight visible to the human eye.

This refraction, or bending of light, is what sometimes lets us see a green color around the sun.

The different temperatures in the atmosphere also play a role, according to Null, a meteorologist with Golden Gate Weather Services. And that is why you’re most likely to spot a green flash on the coast.

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“Right over the ocean, there is cooler air, but sometimes there’s a layer of warm air above it,” Null said. “You have the sun setting, and it’s coming through these layers. It’s getting bent.”

When the sun is on the horizon, it passes through a very thick part of the atmosphere, explained Null, and is more likely to bend. In contrast, when the sun is directly above you at midday, it’s passing through a relatively shallow part of the atmosphere.

The yellow sun is seen setting into the horizon. A background of dark orange surrounds the sun. On the upper most part of the sun, you can see a faint green color.
A green flash photographed in Half Moon Bay, California, on Sept. 30, 2021. (Jan Null)

These different densities caused by warm and cool temperatures create what Null calls a “coastal inversion” when the wind blows from the land towards the sea, refracting the setting sun’s light.

But why is the flash green? As the sun disappears into the horizon, its lightwaves are bent by the atmosphere. The green light becomes concentrated and separates from the other colors in the light spectrum, creating that brilliant green flash.

Although they are quite rare, Null said that he’s even seen “blue flashes” during sunsets, which occur when there’s an even larger coastal inversion as sunlight passes through even thicker layers of the atmosphere.

During a green flash, our atmosphere distorts light from the sun as it sets, and the green rays are what reaches our eyes. (John Jardine Goss/ EarthSky/ Patrick Meyers)

How can I see the green flash for myself?

To have a chance of seeing a green flash in the Bay Area, find a viewpoint near the coast, where the view of the sun setting isn’t being obstructed — and watch the sun as it starts disappearing into the horizon. Remember, because this optical phenomenon happens when sunlight moves through warm air on land towards the cooler air in the water, the places where green flashes are most commonly seen are on the coast.

As for the best time to see a green flash, that will be on a relatively warm, clear day with a light offshore breeze that will help create those “coastal inversions,” Null said.

Null warned that the green flash will last just a few seconds, making it even harder to spot with the naked eye — especially given how our eyesight gets “so degraded” when looking in the direction of the sun.

But even though this can make it “really hard to discern much detail,” Null said this is what you’ll be looking for: “As the sun begins to set, you can see the light from the [the upper edge of the sun], sort of rippling. And it looks like a little bubble of light rises above it.”

And what are your chances of actually spotting the flash? Null said that despite being a meteorologist for almost 50 years, he saw his first green flash only four years ago when he moved to Half Moon Bay.

“When we do optics classes in meteorology training, one of the things they talked about is the green flash,” Null said. “Meteorologists have always been looking for it.”

But for those lucky enough to wait patiently to see this fascinating sight, it’ll all be worth it, he said.

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